The present invention relates to X-ray reflectometric systems that utilize characteristics of X-rays reflected from a thin film layer to determine structural properties of the thin film layer. Particularly, the present invention relates to detector-shield assemblies for such X-ray reflectometric systems.
In X-ray reflectometry of the type described in the above-identified prior art, X-rays are focused onto the surface of a work piece such that X-ray reflections occur in a predetermined angular range. An X-ray detector is positioned in an X-ray reflectometric system such that a relevant portion of the reflected X-rays impinge upon the detector on a sensing area comprising an array of detectors. Each element in the array corresponds to a different angle of incidence of the X-rays in the detector. These elements are resistant to irradiation damage.
In the case where an X-ray detector includes a photodiode array, the primary area is a linear array of X-ray sensitive PN photodiodes, which are well known to those skilled in the art. PN photodiodes suffer little or no degradation when exposed to X-rays. This type of detector also includes an array of readout circuits also denoted as a diode read-out section, which extends preferably parallel to the photodiode array. The read-out circuit performs well-known electronic operations to produce together with the photodiode array an electric signal that corresponds to various characteristics of the impinging X-rays. The read-out circuit includes MOS transistor devices, as they are well known to one skilled in the art. MOS transistors have a leakage characteristic that becomes irreversibly degraded by X-ray exposure. In order to prevent the performance characteristic of an X-ray detector from degrading, this irradiation vulnerable area needs to be shielded against X-rays.
In the prior art and according to FIG. 1, a radiation source 1 provides a focused monochromatic X-ray beam 2 onto a test area 3 of a work piece 12. As is described in the U.S. patent application Ser. No. 09/527,389, filed Mar. 16, 2000, and U.S. Pat. No. 5,619,548, issued Apr. 8, 1997, the beam emitted from the radiation source is typically focused onto the sample by a curved monochometer (not shown). The focused X-ray beam 5 is reflected from the test area 3. A single aperture plate 6 made of radiation opaque material is commonly positioned in the path of the reflected beam 5 in order to limit direct irradiation only to the dedicated sensing area 10 of the detector 8. Unfortunately, the aperture plate 6 creates stray radiation 7 along the plate edges. It is noted that stray radiation 7 is commonly referred to as scattered radiation. With increasing sensitivity of an X-ray detector 8, stray radiation 7 impinging the irradiation vulnerable area 11 (See FIG. 2) becomes a significant factor in the detector""s feasible life span. In the case of a photodiode array detector, the dark current characteristic and sensitivity suffer irreversible and detrimental change after about one hour of operational use of the detector.
Therefore, there exists a need for a detector-shield assembly in an X-ray reflectometric system configured to prevent stray radiation from reaching the irradiation vulnerable area of the detector. The present invention addresses this need.
A detector-shield assembly is introduced that includes at least two apertures positioned and shaped in correspondence to each other, to the sensing area and to the irradiation vulnerable area. A primary aperture is positioned and shaped in correspondence with the reflected X-ray beam and the sensing area. The primary aperture consequently defines the shape of the X-ray beam impinging directly on the sensing area. A secondary aperture is placed between the primary aperture and the detector. The secondary aperture is preferably placed immediately adjacent to the detector.
The apertures may be defined by an opening in a solid plate or by two or more elements suitably positioned to define an opening. For example, a pair of razor blades can be aligned with each other and spaced apart to define an aperture. These structural arrangements, whether a single plate with an opening or separate elements configured to define an opening, will be referred to herein as an xe2x80x9caperture platexe2x80x9d for ease of description. The surface of the aperture plate functions to deflect and/or absorb X-rays striking that surface outside the perimeter of the opening.
The size of the secondary aperture is slightly larger than the primary aperture such that the shaped beam formed by the first aperture may pass without interference through the secondary aperture. Stray radiation scattered or deflected by the primary aperture will be captured by the secondary aperture. Specific dimensioning of the secondary aperture provides for a minimal extension of lateral stray radiation on the detector front. The portion of the lateral stray radiation extension, which is lateral and immediately adjacent to the detector array (impinging area) is called the X-ray half shadow. The secondary aperture functions to reduce this half shadow.
In the preferred embodiment of the present invention both apertures are defined by a single monolithic structure made from a radiation opaque material like, for example, stainless steel or tantalum. Fabricating the apertures as a monolithic structure provides for highest dimensional precision between primary and secondary aperture. The area between the primary and secondary aperture is preferably laterally recessed such that a portion of the emitted stray radiation is laterally dispersed.